Here's another bit of fridge horror-y thought I came up with a while ago (though whether it contradicts anything in FM I cannot remember, I'm sort of half-asleep here):
Gordon Freeman was born in Seattle. He went to college at M.I.T. and spent some time at the University of Innsbruck before getting the job at Black Mesa. As far as I know, those are all fairly temperate climates.
So he moved into the middle of a desert. Chances are he spent most of his time inside the air-conditioned building and didn't have much chance to get acclimated to the heat. During the events of Half-Life, he runs around the desert in a heavy metal suit.
In all fairness, the HEV suit has an automatic medical system, health management, tracking devices, radiation shielding, protection from bullets, weapons management, a long jump module, a HUD, a radio (In HL2, at least), explosive damage protection, crush damage protection, oxygen deprivation protection, Geiger counter, and a flashlight. I don't think it's too out of bounds to believe that it contains a cooling system.
and with the medical system able to readily administer morphine, it may be connected to Gordon with an IV that gives him a saline solution if he is dehydrated.